JAM JAM - 23 days ago 7
C++ Question

On local and global static variables in C++

C++ Primer says


Each local static variable is initialized before the first time
execution passes through the object's definition. Local statics are
not destroyed when a function ends; they are destroyed when program
terminates.


Are local static variables any different from global static variables? Other then the location where they are declared, what else is different?

void foo () {
static int x = 0;
++x;

cout << x << endl;
}

int main (int argc, char const *argv[]) {
foo(); // 1
foo(); // 2
foo(); // 3
return 0;
}


compare with

static int x = 0;

void foo () {
++x;

cout << x << endl;
}

int main (int argc, char const *argv[]) {
foo(); // 1
foo(); // 2
foo(); // 3
return 0;
}

Answer

The differences are:

  • The name is only accessible within the function, and has no linkage.
  • It is initialised the first time execution reaches the definition, not necessarily during the program's initialisation phases.

The second difference can be useful to avoid the static intialisation order fiasco, where global variables can be accessed before they're initialised. By replacing the global variable with a function that returns a reference to a local static variable, you can guarantee that it's initialised before anything accesses it. (However, there's still no guarantee that it won't be destroyed before anything finishes accessing it; you still need to take great care if you think you need a globally-accessible variable. See the comments for a link to help in that situation.)